The present invention relates to a porcelain enamel composition useful in coating a metal substrate, to metal substrates coated therewith, and a process for the production of such coated substrates.
In the production of electronic components directly on the surface of a substrate, conductive and resistive inks made up of metal and glass powder are applied in the desired pattern to a substrate coated with an insulating material. The thus-printed component is then fired at elevated temperature to fuse the materials in the ink and form the desired electronic component. The required firing temperatures generally range from 600.degree. to 900.degree. C.
In view of the temperatures to which the coated substrate is subjected, substrates suitable for the production of electronic components must be able to withstand elevated temperatures without being physically changed. The substrates generally employed are metals such as steel or copper or aluminum. The compositions with which these substrates are coated are generally porcelain enamels but the specific composition of these enamels has varied greatly.
Some porcelain enamel coatings have been found to be uasatisfactory because upon firing, the porcelain does not fuse into a layer of uniform thickness. This lack of uniformity makes it difficult to accurately print circuits on the coated surface.
Other porcelain enamels resoften and reflow when subjected to refiring resulting in distortion or even destruction of the printed circuit.
Another major problem which has been encountered with metal substrates coated with porcelains is poor adhesion. This adhesion problem is due largely to the substantial difference in the coefficient of thermal expansion of the metal substrate and the porcelain enamel coating.
Attempts to resolve these problems have not yet been completely successful.
Examples of specific known coating compositions may be found in U.S. Pat. Nos. 3,904,423 and 4,256,796.